Radio relaying



July l1, 1950 L. E. THoMPsoN RADIO RELAYING 11 Sheets-Sheet yl.

Filed Feb. 5. 1945 ATTORNEY L. E. THOMPSON RADIO. RELAYING July 11, 195o 1l` Sheets-Sheet 2 Filed Feb. 6, 1945 QN INM July 1l, 1950 l.. E. THOMPSON RADIO RELAYING 11 Sheets-Sheet 3 Filed Feb. 6. 1945 L. E. THOMPSON RADIO RELAYING July 11, 195o 11 Sheets-Sheet 4 Filed Feb. 6. 1945 RM O s TP mM M .fish NG MM. .WWK

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RADIO RELAYING Filed Feb. 6, 1945 11 Sheets-Sheet 8 AME L/M il Munn f AAAAAAAAAAA vnvuvvvv' RVi/802 l AMP.

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RADIORELAYING Filed Feb. 6, 1945 11 Sheets-Shea?l l0 A nun - E- -19H0 I .170 kc IN VEN TOR.

L. E. THOMPSON 2,514,425

RADIO RELAYING,

11 Sheets-Sheet 11 July 11, 1950 Filed Feb. e, 1945 BY A frog/viv.

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Patented July 11, 1950 RADIO RELAYING y,I "eland E. Thompson, Merchantville, N.' J.,v asmuysignor to Radio Corporation of America, a

poration of Delaware v, Application February 6,1945, Serial No.v 576,453

1 My present l'invention deals with radio relaying in which the radiowaves employed have freper second. Although many of the features and principles of my invention are described in connection with a radio relaying system operating at 13 claims. (ci. 25o-9) This arrangement is objectionable since each dei y .A modulation and remodulation process is carried quencies of the order of thousands of megacycles very short waves they are not, of course, restricted thereto and have more general application in other systems and apparatus, as will be evident as the descriptio-n thereof proceeds.

Radio relaying is'useful for many purposes.

For example, radio relays may be usedto convey a'program originating in a studio to a distant broadcast transmitter. The relay offers advantages over wire lines for that purpose since the wire lines are expensive to construct and have serious limitations with respect to frequency band 1 Widths which they are capable of transmitting.

In general, unless carefully designed, costly wire lines are built, there` may be serious loss in the quality and fidelity o f the signals kor programs carried by the lines. Radio relaying offers similar advantages over cables-and wire lines when simplex or multiplex signals lare to be transmitted across rivers, bays and other bodies of Water and over mountains,` deserts and other difoult ter'- rain.

When relaying with very short waves having frequencies of the order of, for example,l 3000 megacycles per second the distance of transmissionis limited principally by the curvature of the earth since the short radio waves tend to act like light waves and travel in straight lines without useful refraction or renection' from the Heaviside layer as is the case with longer waves.' This necessitates th-e use of relays spaced about twenty or thirty miles apart depending upon such things as the height of the supporting structure available for the'transmitting and receiving antennas. For a trans-continental radio relay employing ultra short waves it may be necessary, therefore, to use over one hundred relaying stations. Hence, it is desirable that each relay introduce a minimum'of noise and in the case of multiplexing 'upon a common radio frequency carrier crossmodulation and distortion must also be kept to a very low value lat each relay point. Otherwise the integrated effect of the noise and distortion introduced at the relay points Will be such as to make the signal received at the ultimate receiving terminal unsatisfactory and in some cases un-` useable. K

In the case of multiplex signals the received waves at each relay station `could be demodulated to their original signalling frequencies and then used to remodulateA a new carrier. The latter would then be transmitted ion-to the next station.

on with tubes havin'gcharacteristics which are inherently non-,linear and the integrated effect over a chain of relays would be to cause serious distortion and crossfrnodulation. Also, it has been proposedto heterodyne the received waves at each relay'station to some' convenient inter-l mediate frequency and then after amplification to heterodyne the intermediate frequencyv back tos'ome suitable high frequency for retransmiti tal without transforming the received waves into the original' relatively low modulating frequencies. This arrangement, howeven'suffers from thedisl-v advantage that the'heterodyning process used to produce the new frequency for transmittal is relatively inefficient and undue care must be exercised in' shielding the local oscillators and frequency multipliers and other circuits associated there= with to avoid undesired heterodyning actions with the received signal. One of the principal purposes of my present invention is to provide an improved relaying sys-4 tem and apparatus in which distortion and crossmodulation are kept to very low values. To this end I make use of a double angle modulated wave. By angle modulation I mean that type of modulation wherein a characteristic of a continuous wave other than its amplitude is varied in accordance with a sign-al. More specifically, the v.angle modus lation may be pure frequency modulation or pure phase modulationjor a type of modulation having both components. By double angle'modulation is meant a system in which one or more signaling channels are used to angle, frequency or phase modulate a common sub-'carrierfrequency and this common vmodulated frequency is then ein ployed to angle, frequency or phase modulate a wave of still higher frequency of a value suit'- able for radio transmission.

' I have found that double angle or double fre-g quency modulation is inferior to the use of a. single frequency modulation system occupying the same radio frequency lband width or channel, as far as eliminating extraneous and natural noise 'and disturbances is concerned, other conditions being the same. Nevertheless, despite this inferiority double frequency modulation has certain important advantages whenused in a system employing` a number of radio relaying stations, particularly in keeping distortion and cross-modulation down. These advantages will be discussed more fully later.

It is known that assuming the same Vsignal modulation frequency band width the signalI-toL noise ratio improvement of a lsingly frequency modulated radio Wave over amplitude modulation will be equal to the square root of three multiplied by the deviation ratio. The deviation ratio is defined as the maximum frequency deviation in the frequency modulated waves divided by the highest modulation frequency employed. I have found, in the case of double frequency modulartion where the signal band of frequencies is used to frequency modulate a sub-carrier and the subcarrier in turn is used to frequency modulate the radiated carrier, that the signal-to-noise ratio improvement over an amplitude modulation system in which the signaling band'd'rectly amplitude modulates the radiated carrier is equal to 1.23 multipled by the product of the deviation ratios employed in the sub-carrier 'and i'n the radiated carrier. Hence, it is clear that double frequency modulation does not have as good a signal-to-noise ratio as singlefrequency modulation for 'equal transmitted radio frequency band Widths, but despite this disadvantage I have found that double frequency modulation is particularly useful in a radio relaying system employing a number of radio relays. It is for the latter 'reason that the relaying system described herein makes use of double frequency modulation.

In accordance with 'my invention, at a relaying station the received doubly angle modulated or doubly frequency modulated wave is received and heterodyned down to some convenient intermediate'frequency. The intermediate frequency lies within the radio frequency spectrum. This intermediate frequency is amplified and limited and subjected to a single frequency demodulatioii. rIhe demodulated waves then correspond in frequency and in angle, frequency or phase deviation with -the modulated common sub-carrier wave. This reproduced sub-carrier at the relay point is then used, after further amplification and limiting if desired, to directly frequency modulate a new high frequency carrier having a value in frequency suitable for retransmittal.

The foregoing arrangement, I have found, is effective in minimizing cross-modulation .and distortion. My explanation why the foregoing desirable result ensues when using my improved radio relaying system .is briefiy as follows:

In all systems of modulation, cross-modulation and distortion is caused by one or both of two things, unlinear amplitude characteristics of tubes or circuits and unlinear `phase characteris- .tics of circuits. The unlinear .tubes or circuits may be in the modulator, demodulator or amplifier circuits in between. The latter includes tubes and circuits employed `at the relaying stations.

In the case of amplitude'modulation unlinear amplitude characteristics -are by far the most important. `Systems designed for less than one percent overall distortion at one hundred percent modulation may become quite complicated.

In the case of frequency modulation, circuits and tubes having an unlinear amplitude characteristic in the amplifier circuits between the modulator and demodulator do not cause troublesome distortion. However, the modulator circuit-wherein amplitude variationsare rchanged to frequency variations must be linear. Likewise the 4demodulatorcircuit must also belinear in order to avoid cross-modulation and distortion.

There is anothercause of distortion inthe case of angle modulation and specifically frequency modulation systems and that is unlinearphase4 characteristics of vtuned circuits.` In the case vof a simple signaling vsystem making use of a transmitter and a receiver wherein Signals are radlated directly to the receiver from the transmitter, the phase distortion and the cross modulation caused thereby are in general, negligible and of no practical significance. However, in a relay system having a large number of relay stations this cause of distortion may be of primary importance, especially as regards production of undesired cross-modulation.

Consider a frequency modulated signal impressed on a single tuned circuit which is tuned to Athe carrier frequency. At the carrier frequency the circuit acts as a, resistance. However, as the frequency swings lower and higher in frequency, the circuit has some inductive and capacitive reactance. The instantaneous phase of 'the wave is changed due to this reactance. That is, .the current 'in the resonant circuit changes with respect to the exciting voltage. This Vis just the 'same thing as changing the phase `of the modulated wave va small amount. This change vvin phase as the modulated Wave swings in frequency about the carrier is, of course, at the same rate as the lmodulation frequency. If the phase change is linear from the carrier frequency out tothe limit of swing, no ldistortion of the lmodulation will result. If it is noltt linear, distortion of the modulation will Aresu 'This distortion may `be iexpressed in radians. In ordinary tuned circuits, the radians of distortion depend 'upon the ratio of the frequency swing to the rband width of the circuit. The'percent of cross talk produced in the signal depends on the ratio of Vthe radians of distortion tothe angular deviation of thesignal.

Phase distortion alsocauses harmonics of the lower modulating frequencies to fall on or 4near the higher modulating frequencies, and in a multi-channel system 'this produces another form of cross-modulation.

In my proposed relay system, assume for the sake of `further explanation that the received waves are of aform wherein the signaling channels frequency modulate a sub-carrier of one megacycle and this frequency `modulated subcarrier is used to frequency modulate a radiated carrier 'having an unmodulated frequency of 3000 megacycles. At my kproposed relay station the received Waves would be "heterodyned down to an intermediate frequency of, for example, 310 megacycles, filtered and amplied. lThe circuits used here .have a band width `only slightly wider'than the deviation and are used to obtain proper selectivity. This intermediate lfrequency wave, it will be noted, carries double frequency modulations. Because ofthe circuits through which the intermediate frequency wave passes. phase distortion of the sub-carrier frequency of one megacycle results. Also, in the frequency modulation discriminator following the intermediate frequency amplifier, distortion of this one .megacyclewave results from the :unlinear characteristic of the discriminator. .These tWo distortions, however, are on the ,common subcarrier frequency of one megacycle but they do not affect the signaling channels-carried by the common sub-carrier vsince the signal vchannel wave shapes or signal frequency Wave shapesor ulti-mate signal wave shapesfdepend on the rate of change v-of the one megacycle sub-carrier and not upon the shape-ofits amplitude characteristic. If it is desired, the harmonics produced in the one mega'cycle frequency Amodulated subcarrier by phase and other distortions can be i; elimnatedi by: passing? the. wave.y through. a: oneL megacycle band pass lter.-

`kA/i certain .amounttotlteringf in the one vmegafcyclecircuits. atthe relaying station mayvv be.

desirable, .but thel filtering circuits must' be. designed.. with.. care as they. modulating frequencies. `offthe sub-carrier are the ultimate. signaling.

requencies and: the phase' distortion, causing.,A cross-modulation', will result if the phasezchar-` acteristic isv not: linear;

'Ilhedinearity ofI the modulator-:modulating the.. newcarrier at the relayeoff, for.` example, 3010: megacycles by the sub-carrie'rxof:onemegacycla. isanot important. This-.follows since' although dis-tortionistproduced itis distortion of the. one; megacyclebsub-carrier but@v this, again, is; not

the ultimate signaling.- frequency: orarequenciess which:` anatema-keptf-reeotA all forms` of distor.-.. tion if' exact highqualityreproduction isto be. had:

Eurtherr itA will be .noted that. these.. advantages apply only tot a. doublet angle modulatedsystem.y and idornotfollow. in) thev case .ot. double amplitude; modulation. since distortion. of. the amplitude characteristics. of, the intermediate. and sub-v carrier ,wav-esawouldx appear.l ultimately..` in, the nahsignaling frequencies; f Comparable reduction: fon.- cross-.modulation cannot'. be. obtained if: thasystemA is usedV in.. which. the sub-carrier is;,`

amplitudemodulatedby thesigHal andthissubr carrieris then used tomodulate the .,transmitted';

Wave:

Another; advantage of..y the. .doubleY angle or;

doublev frequency..r modulated;y system. described above follows,.fr.om the. fact.; that. most.. of the.v :f

OtherA` objects', advantages,l and features.. of my: e

present invention will be apparent-.as-themore detailed. description thereofr proceeds... Among the; features brieriybe mentioned an` imi-.-

provedl frequency modulating; system in which exceptiona1ly--good.linearity is secured,k therebyenabling multiplexing. atthe. transmitting4 te1f minal. with., negligible cross-modulation and an, arrangement 0f circuitshat receiving points such thathighsensitivity is obtained at certain points wherein non-linear characteristics do not affect the?, ultimate; signal wherein extremev linearity ai',`.1tlreexpense.` of sensitivity is .employed'in otherI portions ,of the vsystem `whereby the loss in sensitvity is more than compensated for by freedemA from, cross-modulation and other distortion'eiects. Fpr. example, in one portion of the receiving, apparatus wherein a swing of plus and; minus 170" kc. may represent the maximum'k frequency swing of a group-otmultiplexed signals, the "discriminatori-or sloping filters-used to convert thisgvaveinto-waves oifvarying amplitudeprior tti-detection may have1 overlappingl resonance curves;v theseparationv betweenv the,- pealrsfofWhiclrfmayr `beeoi theordenoiiplustand.:

mlnusf.- two; million; cycles. Thisimeansefthai-thei waves; are beingl converted:- to rwa-veszof varying; amplitude:A overL a very` small? fraction of.' the: characteristic: of e the: circuitg thus.: insuringii high` linearity in the transformation.

In the.:` detailed'. descriptionA which ffollowsi: and which; is'i given in. connection with: thery accom:-. panying drawings certain values.: of frequencirl have. been. chosen for various: carriers,y channels, etcl` Itis; to benclearly#understoodtthat"these; Valueshave beenl chosen so as.: to lpresent i al tmplcal example-Which may befoll'owed; but obviously." widely diierent, choicesf as.- to* frequency: andi otherva'lues may bemade Hencetherinvene;

. tionris notato be-consideredtas limitedsto. the=va1e1 ues chosen for illustrative purposes.'r

In-..the; accompanying: drawings Figurerlg. llstrates schematically a. transmittinglterminal ifonf ana. ultraY high. frequencyv relayf system. The?, terminal:makesnuse-.of afhigh qualitylvoice chan: nel having, asa.indicated;,.an upper frequencylotf 10,000 cylces although if desired;X thissmay.- bee; raised to .153000 .cycles and! several otheresignalingmhannels rwhich are transmittedto aicommonf; amp-lineras.. side bandsv of.` suitable suba-carriers. Allfofrthefsignals are combined; pre1-emphasized in a suitable network and used tof frequency/J modulate a common sub-.carrier.:having, asillustratech. a-y mean `frequencyfofone megacycle. The.; latter,- .in=y turn, isA used totfrequencyimodulate a transmitted carrier havingi almeanlA frequency -of 3000fmegacycles.. 2

Figure 2;.is a block.;diagram.oflaityplcalrelay@v station employing r theL principles of." myzpresenti invention. It will be noted' that.' thezreceivedfi doublyfmodulatedv.wavesareconvertedtoa.suitable-intermediate frequency, amplied',` and then: subjectedi to;` a single.; frequency, demodulation The i waves resulting.. fromy this.\single frequency.; demodulation are then used in part forxpurposess. of` automatic frequencycontrolfof fthe localibeatingioscillator.. andprincipally toffrequencyrmodu-i late anewglocally. generated carrier.

Eigure 3i is a. block diagram.. of: aareceivingel terminal. This terminal may receive Waves:I transmitted directly 'from the` apparatus of Figurel orfrom airelayingrpointvoristation suchzas: diagrammatically illustrated in Figure. .2.; Inc; the: receiving. system g of Figure, 3vthefl received waves:l are-filrstlV converted tov: a;y suitable inter-1., mediate frequency, amplified, andithen subjected; to a first frequencyedemodulation lcyfadiscrimi-l nator, system having relativelyhighsensitivity. Thelinearity.A ofV this rst discriminator-detector: ls not Eofeparticular importance-as regards kintro-y duction.. of cross-modulation.;y This, of*y course; is;Y equally true.. of the discriminator-detectorsystem aty the relaying points,v as will be clean-- from explanations .to begiven later. These may.;. thereforehave a band widthV only wideenough.

tor. accommodate the signal;` and: can therefore; give the required selectivity. The secondl dis..

suitable vamplifiers andllters .to ultimate signal utilization channels;

Figure '4 is` a` Wiring-diagram y of circuits utilizingr the combinedfchannelsfof 'Figure 1 tovpro-v duce a common frequency modulated sub-carrier:

In order to secure linearityvatv this critical pointl a pair of oppositely frequency:modulatedfoscile-A later-sf.: are used Whichuare! operateduoven'arrelatively small range. VThe outputs of the oscillators are frequency multiplied and combined in a converter in order to produce a sub-carrier of proper mean frequency and desired frequency swing.

Figure illustrates the characteristic. of the pre-emphasizing network used in the apparatus of Figures l and 4; l

Figure 6 is a schematic showing of ahigh frequency oscillator and circuits therefor for utilizing the frequency modulated sub-carrier produced by the apparatus of Figure 4 for frequency modulating a very high frequency-carrier which is to be radiated directly to a receiving terminal or to a relay station such as illustrated in Figure 2. I

Figure 7 `is a more detailed schematic diagram ofthe .first local oscillator and converter employed ata relaying point or at the terminal receiver.

beating oscillator.

Figure '7a-.is a more detailed showing of a suitable arrangement of parts for the-local oscillator, converter and first detector of Figure 7. Figure 7b is a side view of the apparatus shown in Figure 7a.

Figure 8 is composed of Figures 8a and 8b which are to be read `as joined. along the line X--X so that the conductors A' to.L inclusive are respectively connected together.y

Figure 8a is a wiring diagram of intermediate*- frequency ampliflers'and limiters which may be used following the converters 202 and 302 of Figure 2 and Figure 3 respectively. I.v

Figure 8b is a wiring diagram of the first discriminator-detector illustrated at 208 in Figure 2 and 308 in Figure 3. Figure 8b also illustrates apparatus for indicating breakdown in the relaying system.

Figure 9 is' aA wiring diagram of apparatus which maybe used for the oscillator 3| 3, converter and amplifier 3|| of Figure 3- and also for the second discriminator-detector 3| 2 ofv Fig-- ure `3.

Figure 9a illustrates the frequency characteristics of the discriminator circuits930 of Fig-e ure 9; and

Figure 10 illustrates a typical antenna systemwhich may be employed as a transmitting or receiving antenna aty any point in the system where such antennae are required.

In `Figure l, several independent signaling channels are combined and modulate the waves radiated from the transmitting antenna TA to the receiving antenna RA200 of the relay station of Figure 2. The waves received at therelay station are heterodyned, amplified, detected and used to modulate a different carrier frequency wave. The latter is radiated over the relay transmitting antenna TA2|4 to the receiving antenna RA300 of Figure 3. The received wavesy work PN is described more fully later in ,con-` nection with Figs. 4 and 5.

Oscillator may` operate, ,by w'ay, of example,

Figure 7 'also illustrates circuits for` automatically frequency vcontrollingthe first quency.

at an unmodulated frequency of 10 megacycles.` and oscillator |02, for example, at an unmodu-- lated frequency of 11 megacycles. The'outputs scribed later, the outputs of oscillators 25l and |02 are frequency multiplied before beingcomf.

bined in converter |00.

It will therefore be lapparent that eachoscll-v lator, in the arrangement of Figure 1, need swingV only half as far as would be the case if only onev oscillator were used to produce a given amount:

of frequency modulation. As a consequence, dis-- tortionis reduced since the working range of the voscillators is made smaller and over the smaller;Y

range they can be made more linear in action.

Cross-modulation between channels is therefore; greatly reduced. Furthermore, such an arrangef; ment serves to reduce humr due to filament heat-Y.

ing or ripple in the plate voltage supply.

Each of the channels A to F, inclusive, s'ad justed in amplitude so thatthe deviation ratio, for the frequency modulation produced by each; channel in the output of converter |00 is unity,l but the total maximum swing produced by alli of l'the channels is plus and minus 1'70 kilocycles as indicated in the drawing. In other words,4 channel A produces a maximum swing of 10kilocycles, etc.

signals are instantaneously additive, the output of converter |00 is then being modulated plus and minus kilocycles.

the pre-emphasizing network PN, as a result of which the signalling channels have substantially the same signal-to-noise ratio-a desirable feature in multiplex signalling. i

The frequency modulated output of the con-- verter |00 is a beat of one megacycle plus and..A minus 170 kilocycles and is used to frequency` modulate a second frequency modulatedvoscil lator |04 whose mean unmodulated frequency is 3000 megacycles.

As a result the wave radiated over the trans-c mtting antenna TA of Figure 1 is a 3000 mega-* cycle carrier having a maximum deviation of cycles.

More specifically, with reference to the channels A to F, inclusive, channel A is a high quality.l voice channel containing all frequencies in the band from 30 to 10,000 cycles.

The high quality voice signal is picked up by"l microphone 2, amplified by amplifier 4 vand sent through iilter B and another amplifier 8 to,V the,

combining resistor 23.

Channels B to F, inclusive, are low quality; Voice channels each passing through the first` amplifiers 4B, 4C, 4D, 4E and 4F, different voice signals lying in the band from 30 to 4000 cycles. These amplified signals are fed tothe modulators ,i

The foregoing ad-- justment and operation are providedby use of.

12B .to 12F, inclusive, supplied with oscillations from separate oscillators B to 10F, inclusive.

The output of the modulator 12B isfed through a .filter MB which passes -only the lower side band. Similarly,llters MC to MF, inclusive, pass only the lower `side .bands produced, respectively, in modulators .I2C to I.-2F!, inclusive. In the case of :filter 14B, 4the band of -frequencies passed on to amplifier 15B occupies .the range from 12 to 16 kilocycles.

Similarly, the .lower iside vhand .filters MC to MF, inclusive, ypass .on to ampliers 16C to IBF, inclusive, 'the lower iside bands .derived from the immediately precedingmodulators 12C to V12F, inclusive. The -frequency band .passed .by each side band illter ris indicated in .Figure .1. Thus MC passesZO-Zkilccycles etc.

The outputslof the lower side band .amplifiers I-SB to .IER inclusive .are .combined .as vindicated andied througha .band pass lterf'll toamplifier 2.2, which s .made as ylinearas .possible to prevent cross-modulation between channels. The output .oiamplifierZZfis combined .with theoutput of the high .quality channel ...f-rom amplifier 8 in resis- .tor 23.

The resulting voltage ,across .resistor ..23 -occu pies :ahandof ,frequencies .irom-30 yto 48,000 cycles .and this .band .is .-,fed through transformer 2,4 vto .the .oppositely frequency modulated oscillators Y2.5 and izhaving, respectively, .unmodulat ed .carrier frequencies .of .ten .and yeleven mega- .cycles y.The;-arnplitu.de ofthe voltages fed .from each channel .is adjusted, -aswill .be .more Afully explained later, so that each .channel produces .frequencymo dulation fin .the A.output :of [converter i110 withla .deviation ratiof unity. Thuschan nel Ahaving an .upperirequencyof .101,000 .cycles deviatesthe.outputof .converter Milian amount of rplus |and .minus f10,000.c.ycle`s. Similarly, Vthe maximum lamplitude .of voltage -ied through channel .'fB to .resistor .produces a y.deviation of plus Y.and minus l-kilocycles and, similarly, .for maximum .amplitude of .input #channels .C, D, E

.and .F produces respectively, deviations fof plus and minus .2.4 kilocycles, .plus and -minus 32 kilo- .cycles-plus and minus 40 .kilocycles and .plusfand lminus eykilocycles. Whenell of'the channels are fully modulatedwand when-theyfare `all ad ditive or sinstantaneouslyin .phase 'and -of the same lpolarity, "the beat -between voscillators .2.5 and *i022 appearingiin theroutputfof .converter l 0.0 .is deviated afmaximum-oi plus :and .minus .L70

kilecycles.

.Theffrequency zmodul'ated-'outputfof Al ilgname .ly, Iafdifferencefrequency of one snegacycleiplus `and minus i170 :ikilocyles isi-.picked oi andused .tczirequency modulatexthesecond frequencymo u-latedoscillator-'104itiperatingfat an'f.unmedulated .carrier ffrequency lof 1.3000 t'megacy-cles.

The l'deviation .ratio fof .the modulated Waves appearing in theoutputfcircuit:oftheesecond-fi-requency. modulated oscillator -il04'fis unity or remore, as. desired,.as.a:result of which-the waves radiated overthe.transmitting.antenna TA have formaximumdevi-ation, ajrequency of.3.000 megacycles plus andfminus-l megacycle. .Alargendeviation ratio may `lcreused, in which .case .-.theradiated waves .wouIdbe,.for example, l3000 megacycles plus and 'v minus "'3, fully modulated. Y

'The wavesradiated from'ithetransmitting antennafiA O'TFigure 1 'may -loe received directly bythereceiving a-pparatusofiligure 3. 'Ordinarily, V"however, such waves `wmildbe 'radiated to 'the'receivingiterminal"-byMwayof .onefor more "reor more megacycles when laying points, such as .illustrated in Figure 2, The .waves would vloe received at the relay point. at one frequency and re-transmitted to the nexty point ,fin vthe .system at some different frequency so as to avoid feedeback or singing at the relay station.

Iny the relaying system illustrated in Figure 2, .the waves are ,picked up or received -on a receivingantenna RAZENI. The received waves are heat down in frequency in .a converter ,circuit 202 With waves from .a .local beating-oscillator .204.. The intermediate frequency produced `vmay. .be 30 megacycles .plusand minus 1.0 megacycle. The Waves of intermediate frequency are amplied in an intermediate lfrequency amplifier .2116 and then fed to a discriminator detector .208.

rlhe .actionof .the discriminator detector issuch as to .produce a =valave of .one megacycle .plus .and minus 1,70 ,kilocycles .corresponding to .the output of -the .converter .I.00.,of Figure 1. This wave is limited .and arnpliiled in .appropriate .apparatus Zlandthen used .to frequency modulate oscila later .2|:2 whose unmodulated frequency maybe 3010 .megacycles By `adjusting the amplitude .of the output .of amplifier 210 the waves radiated over the .transe mitting .antenna TAZM of vthe relay pointof Figure .2 may be made .3010fmegacycles plusand minus .1.0 .megacyole l i .The .relay system as .described in Aconnection withFigureZ has .denite .practical advantages over anarrangement.wherenthe received ,waves are .demodulated .down .to the .original signals and `the latter ,are .used vto remodulate a .newly generated local wave. VIt willbe noted .that .reproduction .of the 'original signaling waves and amplification of the same `in acornrnon amplifier, prior to their .use Yfor remodulation ora newly generated carrie-r, will introduce undesirable cross-modulation. This .follows from the Afact that the .prccesspf .demodulation and amplifica.- tion ina .ccmmonfamplier `takes .place .with ap.-v paratus having non-linear characteristicsand'it is .these non-linear characteristics ywhich .Cause .the cross-.modulation difliculties. However, even with non-linear demodulator 1 and modulator f cir cuitslandpparatus, the relaying system of Fig-1 ure .2Y will .not introduce .cross-'modulation Itis tolse noteldlthatin the `system of Figuliez their)- termediate frequency ,ampliiier 206 Amay .also .he l

a .high degree .oif Yamplication is Asecured `with y' amplier 306.4

The .output `of the iirst discriminator detectQr l 308'is .they onemegaycle plus and minus 170 kilo- `cycle wave Acorrt-:sponding vto the Aoutput ,of the converter [00 of Figure 1. 'The output of the first discriminator .detector .30,8 of Figure 3 is then. amplified and limited inamplier limiter 3.50. Hence, 'it will be observed that the for.. wardjportionof 'the apparatus of Figuret Yfrom 'RAB 0'0 fto .the limiter g3 l 0 ,jis substantially identi;

lcal` tothe apparatus"betwe'enRAZ and limiter 11 2100er Figure 2, as a result of which economy in the design and flexibility in the use of the apparatus are secured. p

The output ofv the amplifier limiter 3I0 of Figure 3 is fed to a converter 3|| supplied also with oscillations of a frequency of, for example, twelve megacycles from oscillator 3|3. The upper beat of converter 3I| is fed to discriminator detector 3|2, in the output leads 3|3 of which appear a band of frequencies from and including 30 to 48,000 cycles corresponding to the band of frequencies fed through transformer 24 of Figure 1 to the frequency modulated oscillators 25 and |02. i Of this band of frequencies filter 38AR, to which the band is fed through amplifiers 34, 36, passes the high quality voice channel A containing waves lying in the band of to 10,000 cycles. These waves are amplified in the amplier 40AR and fed to a loudspeaker or earphones A. The other frequencies corresponding to the lower side bands of channels B to F inclusive of Figure 1 and occupying the band from 12 to 48 kilocycles are fed through band Vpass filter 44 and amplifiers 46 to 54 inclusive to the filters 56 to 64 inclusive. e

f Filters 56 to 64 inclusive pass bands of frequencies as indicated in Figure 3, namely, filter `56 passes 12 to 16 kilocycles, filter 58 passes 20 to 24 kilocycles, filter 60 passes 28 to 32 kilocycles, filter 62 passes 36 to 40 kilocycles and filter 64 passes 44 to 48 kilocycles. The outputs of filters 56 to 64 are combined in the converters 66 to 14 with oscillations from local oscillators 61, 69, 1|, 13and 15 operating, respectively, at 16 kilocycles, 24 kilocycles, 32 kilocycles,l40 kilocycles and 48 kilocycles. Each of the filters 16 to 8471s designed to pass a band of frequencies fromy 30 to 400,0 cycles, as a result of which in the amplifiers 86Uto .94 inclusive the originally transmitted signals A to F inclusive appear. These waves are individually translated, as indicated, by the earphones B, C, etc.

Also it is to be noted that all of the channels need not be voice channels, but, if desired, some ofthem may be telegraph channels, some voice and some of other types, such as facsimile and teletype channels. Thus, as a possible alternative channel A may be replaced by twelve telegraph channels, the separatetelegraph carrier f tones of which may occupy the band from 465 to 22,95 cycles, each tone channel having a width of 170 cycles.. Thus, the first telegraph channel may be v designed for a tone carrier of 465y cycles with asignalling width of plus and minus 85 cycles, the second tone channel may use a tone carrier of 595 cycles with a cycle widthof plusand minus 85 cycles, etc. j I

In addition to channels A-F inclusive,. of Figure l, a service channel SC may be provided. The output of the service channel pickup microphone may be amplified by the service channel amplifier SCA and switched directly, by means of switch SCS, to frequency modulate oscillator |04. `Preferably, amplifier SCA passes a band of approximately 0-5000 cycles and the amplitude of the modulating voltages is adjusted so as to produce, for example, a maximum swing of 15,000 vcycles in the output of oscillator |04.

, As indicated in Figure 2 the service channel band may be filtered out by filter SCF and taken ,from line SCL for use in earphones, or theoutput of line SCL may be fed by patch cords to the service line input SLI to modulate oscillator 2|2. In Figure 3 the service band of frequencies may 12 be taken directly from the output of the first discriminator detector 308 through line SLR and utilized as found desirable.

Figure 4 is a wiring diagram of a preferred form of apparatus between transformer 24 and the 3000 megacycle frequency modulated oscillator 1040i Figure 1. Figure 4, in other words, illustrates in greater detail the frequency modulated oscillators 25 and |02 and converter |00 of Figure l. Specifically, invFigure 4 the Wave band representing channels A to F inclusive andrunning fro1n30 cycles to 48 kilocycles is fed through'the secondary of transformer 24, pre-emphasis networks`40l, 402 tooppositely control the conductivities of reactance tubes 403, 404. The reactance tubes oppositely vary the frequencies of oscillators 405, 406 which, by way of example, in the no signal condition may be set to run at frequencies of, respectively, 8.5 and 8.83 megacycles. Hence, when oscillator 405 increases in frequency, oscillator 406 decreases in frequency and vice versa.

The output of frequency modulated oscillator 405 is fed to a frequency tripler 401 and the output of frequency modulated oscillator 406 is fed to a frequency` tripler 408. The outputs of the two triplers 401 and 408 having unmodulated frequencies of 25.5 and 26.5 megacycles are combined in the converter or mixer 100, corresponding .to the converter |00 of Figure 1, to produce an unmodulated sub-carrier of one megacycle. The latter is fed through the output leads IOI to the 3000 megacycle, frequency modulated oscillator |04 of Figure l.

It should therefore be clear that the oscillator digrammatically shown at |02 in Figure 1 includes oscillator 405, reactance tube 403 and tripler 401r of Figure 4. Also schematically shown oscillator 25 of Figure l includes oscillator 406, reactance tube 404 and tripler 408 ofy Figure 4.

To go intogreater detail concerning Figure 4, a monitoring jack MJ, for monitoring purposes, is connected to the primary of transformer 24. The secondary of the transformer is shunted by loading resistors LRI and LR2. The pre-emphasis networks 40|, 402 are composed of condensers 409, 4| 0 having a value of 220 mmf. each connected in shunt to resistors 4| I, 4I2 each hav- Aing a resistance of 150,000 ohms. As a consequence, the pre-emphasis networks will be found to havev a characteristic which is substantially fiat over the range from approximately zero to 10,000 cycles and then rises linearly with frequency from approximately 10,000 to 50,000 cycles as shown in Figure 5. In this Way, the outputs of amplifiers 8 lto IGF inclusive of Figure 1 may be adjusted to the same value and the pre-emphasis networks 40|, 402 will operate to produce the accentuationsv which will give the desired deviation ratios mentioned previously in the frequency modulated output of converter I 00.

The outputs of pre-emphasis networks 40 I, 402 are fed through volume controlling potentiometers 4I3 and 4| 4 and through radio frequency chokes 4I5 and 4I6 to the first grids 4|1, 4I0 of reactance tubes 403 and 404. Radio frequency by-pass condensers 4I9A and 420A are provided in order to further insure absence of radio frequency currents from the pre-emphasis networks and preceding apparatus. The cathodes 4I9, 420 of the reactance tubes are connected in parallel and to the common cathode return resistancecondenser circuit 42| consisting of resistor 42|A and condenser 42 IB connected in parallel. This common cathode return serves to maintain conil?. fstant fgridbias ron fthe' rea'ctance ftubesz'sirrce"` they are oppositely modulated. This, therefore, avoidsrfacertainfamountiofdegenerativefeedback at'low frequencies lyfliich would otherwise foccur unless the lay-passing condenser -f42'1B 4is made very large. y

``A voltage doublingrectier`f422`is connected 'through' highf-resistorr4 2 3B,Sswitchi 423C, :byepass condenser 423 to the potenticmeterr4I4,'-as indicate'd; for `monitoring purposes, it`being noted that i in "this 'connection "a Aimilliameter 0423A :is provided. It'is to' be noted lla1sovthat :tthe meter 423A mayfbe connected to'there'sistor *banks-424A -for indicating voltages :and currents infvarious "parts'fof lthe 'circuits,'.as 'will be evident'ttothose skilledin'ithe art. Bymeansof theimeter Man'd rectier 422 the voltag'e'input Itothe reactance Vvtubesm'iaylbevdeterrriined 'K and :adjusted so Tas" to jzproducei-athe desire'dfrequencydeviations i in the osillators140l5,2406.

1Quadraturevcltageris'ieditorzthegrid:4 l 1 from thecplateaof oscillatortubeewethrough theainetwvorkeiconsisting r of @blocking fcondenser 424, -re zsis'tor .425 fandicondenserr425. ;Asa consequence, stliezplatemircuitzof.reactance tuben 403 `appears as sa rvariableainductance A'to the platecircuit of os'- cillator' 4.0.5. by-:passingf condenser 1412-1 :havingi' a 'neelligiblefeiectzinthisiregard.

V,F.Eube #40.5 f acts :as an woscillator because rthe ftuned p1atetcircuit428 -is'coupled backonto the grid 429 through ticklerfnoil-2430 yand by-jpass. condenser^f43 I "ZThe-screenf grid 1432 A4of tube'illii -isrconnected directlywto-the f-plate l`oftleat tube, yas :indicated raxsra'iresult of ywhich*tube429wacts` es- -fsentia-llyasrartriode.

Other frcircuit. elements rof the` reactance tube 1403,fsuch as Vchoke' 433iV for,supplying?plate-voltage, :bye-pass @condenser --434 :and `-w/oltage droppine. resistor n435 :landfsimilar-` elements for the oscillator 1405, farefbelieved to vbe understandable frcmsthe Y A drawings and need not be` disousseddn detail.

`Sincewreactancatube A04-andfosci1lator 406 are isim-ilarfin alliessential respects toreactance Jtube A03 andvosc-illator tube405;fthereis no need tor go .into Y .detail 1- concerning -zthe -,corresp.onding r circuit .elements which have .iust-beendiscussed. It may be sta-ted,- however, that 404 --alsorappears .as .a svariable inductancefacross -the .circuit f including tube406,.but since sional-voltages. cause .tube 403 to -V:become more ccnductiyeand r404 less.. conductive land.viceyer-sa, the frequency vofaogoeration 'of .the oscillators- 405; and are `-varied .oppositely. Hence, y'for given frequency swing ,thefeilfective lraneeoyerewl'iich,y eachxoscill-atorlis Vvaried-is made rsmaller, .resulting .in -f greater linearity iin opera- :.tion. .Theextent of .this I:ra'ngeis `further l'effectivelyfreducedby havingathese oscillators. namely e405 :and .406 operate the sfrequency ltriplers '-40.1 408. The Vtriplers server-to feieetivelytriple the .'deviation.produced inttheeoscillatorsfand, .-hence, when the outputs of the triplers are beat together mixer' 400, .the .outputof'tlfie-.1fnixer-Millv contains :a: deviation which: is of-:a Valuey :corresponding 'to threetimes the diii'erenceinfdeviations'fof 'the oscillators i405, 4056.

Th-e: triplers @401, 'rl408fare fe'dy @from theY oscilla- "ltor's throughfcoupling condensers433; 4134A. 'The itriplers arei overloadedva-cuum tubesiand, hence, 'fb-y' appropriate fltun'in'gf of the r'late -icircuits 435, 430,'the third, 'for for that matter, 'any desired narmoniemaybe 'p'icked dif. Theseou'tput lcirucuits lmay?fbebreadenedby the' use of resistors 431,

438 and tuned by meansro'fffth'eyariableiron-cores 439, 440. Such yar-iableiron'core tunin-g'isfalso :and 436 1'is'fed`zthrough:condensersf445 .and-:446k

tothe .@grid1441 nof the` mixer for detector '2100. Consequently, 1 if Etlne'foutputcircuits 435-fand--436 areitunecl torlthefthird harmonics."of:th'eirprecedingmespective oscillators T-405, 406, and .assuming oscillators 405 .andf406 fto be operatingV` atfiand 'S'SSSine'gaoycles iin the rabsence of input at transformer-I 24,1- th'enthe wavesappearing inK thei.zout put. leads lill r.will haweiairequencyzequalsto Substantially 'oneinegacycle As before explained, .presence :of :signal in- .transformer 24 will cause .the frequency of the waves .appearing lin leads Ilm' to Varytas (desired, dependingupon-the' adjustment of potentiometers 413 and 4M. "These adjustments tinay 'Tebe imadef :such that 'F.this (one meeacyc-le Wave Aappearing .in leadsli0l is fre- :quency modulated-@lus'and-minus 17,0 #kilocycles .Wlienfallfchannels :'PrtoF'4 yinclusivev are supplying maximumzamplitude' voltages tothe :ampliersi22 fofilligurel,

ATo isurnmarizevwith'reference to Figure .4,ethe Ibiariiiof'.'frequencies from 30 to 48,000 -`cyclesfiis 'resemphasizedrbythenetworks 40|, .4'021so'tl'i`at thefinputfof thefreactance tubesc403,-404fis flat mfer the frequencyrrange from :.30 to' 10,000rcycles 'and 'rrises .linearly from `10,0001lto 148,000 :'cycles. This :characteristic:isindicated )in .Figurez` The volume fof time' inputto'the reactanc'e tube'fmodu- Aflatfms-41li3, 404 iswcontrollei rbyfmeans of`poten tiome'tersr t3 444. The .'reactance` tubes "403 ,i404 fserve atto ioppo'sitely modulate ythe lfrequencies 'of oscillators `405, Av45116. :operated beyond saturation, desired harmonics Amay :be picked'out by the tuned "output :circuits -oif the frequency xmiltipliers VF401, 408 @and fthe dey-iationWilt bein'creased 'according' toi the .order y'of :the 'harmonic chosen The houtputs Lof #the firequencyfmultipliers "401, -408 'are :beat together in lafmxer tzandfthe outputrof the mixer ,or detector titi! is,'therefcre, Ya frequencymodulated nnauecliaving very llinearirequency deviationfwith 'amplitude :of`r input appled'a't the reactancetubes. .Sueh-:actionis'highly important in order toavoid undesirable cross-modulation of "the signalingv vchannels. The@ outputof A'mixer Y L00 may be fed th-rougna coaxial line having'aSgrou-nded'fouter metallicitubeeand fan innerY conductorito:fthenext :stag-e -of the isystem, namely, apparatus `@1041er In yconnection `-With :the l-reacta'nce Etubes zdf l Eigureellnsueh as, fcr-examplegtubeslllfit:isito ibe noted :tha-tithe quadrature `vvoltage developing -condensers:such as--42-5 should be-madeVar-iable so that quadrature yvoltage :feed-back may be controlled and reduced to any desiredextent. Also by '.adiustment yoithe quadrature condenser, nicnas/24, the apparatus may be. operated `With optimumlinearity. `As.set.upfeach:oscillatonsuch as tubes `405, 40%...andits corresponding reactance tube, namely, 2503 an`d`404, is substantially linear over va 'range of loperation of .approximately .il-290,000 cycles. Of thisrange only a relatively sznall 'portion'isnsed 'for/example, .approximately -fkilocycles'in ordertoinsureextreme linearity fof 'frequency modulation lor *frequency shift" with theapDlie'd modulating voltages fe'd to tliegids "of the reactance tbes 403, 1404 `fromp'otene {tiem-eter l 3, s4l 4. The precautions in thesecurffing of lextreme linearity rare "observed inord'er indicated.'fortheeplatefcircuitsf th'efoscllators 1725 Linthe"transmitting'apparatus Wheref-crossem'odulation due to non-linearity ywill'tend to take place to the greatest extent. f

The common band-pass amplifier 20 and common amplifier 22 of Figure 1 should be designed so as to have a wide flat characteristic of from 10,000 `to 100,000 cycles to not only avoid the introduction of undesirable distortion and amplitude changes, but also to accommodate additional channels, if desired. Further, in order to minimize distortion and cross-modulation, amplifier 22 of Figure 1 should be operated on a linear portion of its characteristic. Amplifier 22 may include degeneration so as to improve linearity, if desired. Typical degenerative circuits and principles which may be used in connection with amplifier 22 are to be found in such patents as Black Patent 2,102,671 and Oman Patent 2,255,804. Also, it will be noted that the reactance tubes 403, 404 of Figure 4 are operated over a relatively small range which is substantially linear so that distortion and cross-modulation are minimized. The circuits of the oscillator tubes 405, 406, such as the tuned output circuits and in particular the tuned output circuits of the triplers 401, 408, are made sufficiently broad so as to be substantially Wider than the frequency swings of the currents fed to these circuits. The output circuit 435A of tripler 401 is broadened by resistor 431 so as to be flat over a band which is substantially Wider than the frequency swing appearing in the output circuit of tube 401. For example, the characteristic of circuit 435A should be fiat over a band of 400 kilccycles for a frequency swing of i-75,000 cycles. The output circuit of mixer l04 should be fiat over a band 800,000 cycles wide where themaximum frequency shift of the Waves appearing therein is i150 kilocycles. In this way, phase distortion is kept to a very small value thereby further reducing the cross-modulation which may occur due to the unlinear phase characteristics of the circuits. In other words in order to minimize cross-modulation due to phase distortion, it is preferred that the frequencyswing used in the circuits up to and including the mixer |04 be well within the flat portion of the amplitude frequency characteristics of the circuits involved.

A further advantage of the modulating system shown in Figure 4 arises from the fact that if the cathodes are energized with alternating currents and if the anodes or other electrodes are supplied with imperfectly filtered, rectified commercial sixty cycle power current, the variations in excitation will tend to cause the oscillators 405, 406 to change in frequency in the same direction. Hence, these changes in frequency tend to become self-cancelling in the mixer |00.

If desired, automatic frequency controlling circuits may be used in connection with the modulating apparatus of Figure 4. In that event, a partof the output appearing in lead IOIA may be divided down in frequency and used to operate a reversible motor, in turn operating a tuning condenser of one of the oscillators 405, 406 such as the tuning condenser 490 of oscillator 405.0r the plate circuit tuning condenser 492 of oscillator 406. Or, if desired, both tuning condensers may be actuated by the automatic frequency control motor in such a way as to bring the beat in lDIA to its desired mean value. The manner in which the tuning condenser is varied by the frequency divided Waves may be that arrangement' as described in Morrison Patent 2,250,104. f

Also if desired and in the alternative, automatic frequency control may be applied to one of the reactance tubes 403 or 404 by first heterodyning dwown a part of the output appearing in lead i0IA with waves from a crystal controlled oscillator and discriminating and detecting the resulting beat for use in one or both oi the reactance tubes 403, 404. This arrangement may follow the principles and apparatus described in Crosby .Patent 2,279,659. Or, automatic frequency control, using part of the output appearing in lead 101A and a connection to the reactance tubes for that puropse, may be employed using the circuits and principles'o Schaeffer Patent 2,274,434.

It should be apparent, therefore, that several advantages flow from the arrangement shownln Figure 4. For a given frequency deviation desired in the waves appearing in line |0l,`the oscillators 405, 460 need be varied only over a. relatively small range. Hence, extreme linearity is secured in this portion of the apparatus. This is desirable for, otherwise, departures from linearity would produce relatively large amounts ofcross-modulation. Furthermore, the arrangement of Figure 4 balances out and substantially reduces hum due to ripple in the plate voltage power supply and A. C. heating of the cathodes of the various tubes involved.

In Figure 6 there is shown a form of high frequency oscillation generator which may be used at |04 in Figure 1 and at 2|2 in Figure 2. Figure 6 also illustrates circuits for producing frequency modulation of the high frequency oscillator.

The oscillation generator of Figure 6 comprises an evacuated container 600 which may be of glass or metal, within which are contained a heated cathode 60|, a screen electrode diagrammatically illustrated in section at 603, a cavity resonator 604, and a disc-like metallic anode or electron receiving plate 605. The cathode 60l is externally grounded at 602. The cavity resonator 604 is made of metal and consists of a metallic cylinder 606 having metal bases 601, 608. Mechanically and electrically fixed to the bases are the internally protruding sleeves or tubes 609, 610 separated so as to have between them a gap 6| I. The tube 600, cavity resonator 604, sleeves 609. 6|0 and plate 605 are shown in cross section.

Actually the cavity resonator may have different dimensions and be proportioned differently, than as shown in Figure 6. The distance between the bases 601, 608 may be equal to or less than the internal diameter of the cylinder 606, as shown diagrammatically in cross-section in Figure 6a. Also, the bases may be dished in and the cavity resonator have the toroidal or doughnut shape shown in cross-section in Figure 6b.

The anode 605 of Figure 6 is maintained at a negative potential of the order of volts with respect to ground by means of lead 6I! connected through resistors 6I3 and 614 to a suitable source of potential SI5 by-passed to ground by means of the by-pass condenser 6|6. The cavity resonator 604, together with the grid 603 connected thereto, is maintained at positive potential of the order of +300 volts, for example, with respect to ground by means of lead 6I1 connected to a suitable source of potential 6|8 bypassed by condenser 6|9.

As a result of the foregoing construction, electrons emitted from the cathode 60| are attracted toy andpassthrough the hollowportion of tubes. The electrons then approach'the negatively charged.`

609 across'A gap 6H and through; tube 010.

is'also dependent, to a certain extent, upon'the" voltages applied to the variousY oscillatorv ele-` ments. l

Output venergy is taken from resonator 94 by means'of conductor S20 coupled by means of the inductive `loop 621- to the space within thecavity resonator 001i.-H Conductor 620 isV suitablyv shielded by means 'of theexternally grounded metallic coaxial conductors (52m,y 622. Y The highfrequency conductor 021i leads to and excites the `transmitting antenna TAof Figure 1 or the relay retransmitting antenna-TA-lll of Figure 2.

When-the. oscillatorin Figure v5 is used inthe transmitting arrangement'ofFigure l, it is modu lated by the'. output of theconverter 'or mixer lll oli-Figures 1 ande. The output ofmixer it@ is fed throughconductor:v Ilila to'the'anode 4circuit of anode 605 vof Figura 6.v In the `case of the transmitter of' Figure l conductor lilla will carryk a frequency modulated wave -of one megacycle having a maXi-m-umirequency.deviation of i170 kilocycles, according.y to the example chosen.

The rWaves. inconductor lll-la, referring to Fig@ ure 6, are resonated in the parallel tuned circuit 623 comprising coil-.624, towhichconductor lilla isV variablytapped at tapping: points 25, and condenserV 620. n y ened by use of atloadingvresistor 02'! connected inshunt to the circuit; By means of variable condenser 628.,..the frequencyvv modulated waves' appearing inline lilla are applied, in controllable amounts, to theplatell.. As a consequence, the output kof the oscillator of Figure 6, appearing. in Ylead 62.0, isfrequency modulated to an extent which may be'lcontrolled primarily by adjustment of condenser 628,'and secondarily by adjustment of tap .525. s

Since the negative voltage applied to the lead 6.!2. .is .fed through resistors 013, 614'. which may, by way of example, be 22,000 and 180,000 ohms in value, respectively, leakagev .of vthe wavesy appearing in circuit=623 to ground through lead lil'l is effectively prevented'.

For .monitoring and -radjustment purposes, a portion of the high frequency waves fed through condenser 628. to the plate 6054 may be -shunted through high frequency by-passing condenser 029 to switch 63,0. The latter, in its upper contact position 63| feeds the recti-er 632 to the output of which is connecteda suitablemeter. The rectified output of rectiiier 632 will indicate the voltage applied to plate 625 and will be a measure of the frequency deviationin the oscillations generated bythe oscillation generator and fed to the output transmission Vline 020.-

The service channel is fed through switch .SCS of .Figure 6, which corresponds to switch .SCS of Figurel, across a potentiometer 531i. For modu lating the highrequencyfoscillator of Figure 6 with the service channel voltages, the latter are fed through tap 635, audio frequency lay-.pass

condenser 636, across resistor 6 mand through resistor '6l3 and vlead 612. to .the anode 605 of the oscillation generator. By throwing switch 630 to the lower position 63.1 theextent of the frequency modulation` producedbyft'he service. channel may The tuned circuit 023 isfbroadz 18 then bev-'measured .by noting the reading onmeter. Mwhich will then be actuated by rectiied service channel voltages. For aurally monitoringthe service channel an amplifier 638and earphones *i 639 are provided, as indicated..

It is again repeated that-all values of requen=` cies, resis'tances, voltages,.etc. are given. as illus.-

trative` or. typical only..and,-` therefore, it .is to clearly understood that allinventions described herein with reference to: all figures of thev draw# ings are notA to berestricted-to such values.

InFigure 6 the la-mentheatin-g voltage source for cathodef r(itl is illustrated. to be a battery butthis battery maybe replaced by a transformerl supplying suitable'alternating.voltages to the lila,-y ment forheatingthecathode to an electron emis-r sive condition. for. the cavity and, plate.v may be replacedl by potentiometers. supplied" withV rectified commercial'60' cycle current. Such alternating currents for exciting thev i'ilamentand' the ripple in the rectified voltages may produce cycle vand i120" cycle/frequency modulation of the output of the' oscillator of Figure 6l This hum willtherefore appear in the service channel. I'twll not appear,l however, in the high quality channel A or in the channels B. to i"inclu`sive;. since. such low frequency modulationl is: eecti-vely ltered out by the selective circuits for those channels.

This' filtering action fellows since therel isi-a substantial separation in frequency between the rst signicant: side Vvbandsproduced by the sub# carrierl in the-outputloi" converter'illand the side bands produced by the low frequencypower modulation. The low' 'frequency power modulation is produced -by the'160 cycle'heating'supply' or harmonics oflcycles representing ripplefin thelrectied power suppl/yi Thisundesired lowiirequency modulation. may also Abe y produced :by undesiredimechanioal vibration. i

It' isY to' be noted" that-oscillators of theftlype shown in Figure 6V Yarea peculiarly susceptibleto thisflow irequency'- type -oiirequency modulation clue tomechanical w/'ibrationA `orthe use of4 im# perfectlyfltered 'rectified power. or dueto use' of alternatingcurrentoperatiOn -oithe cath'- odes; It isvone feature lof inventionithat .the type .of modulated' oscillator. lshown in- Fig-urac; which is particularly susceptible to frequency modulation due to fimperiectly filtered,- rectiiied power or' tothe use of `altern-ating: current on'the cathode, canbelusediwithoutldisturbing the sig-nal;

Incidentally if desiredto transmit a: i gle Vchan-nel, for example high Y'quality channel A alone, ainpliiierj`22 -offligL` l wouldv be switch outf' of` circuit softhatacrossl'resistorf 23, only Voltagesrom channel A ors amplifier/i3 -would be set up. Channel A would be adjusted-so veis-te produce er full deviation of plus and minus-- kilocyclesin the outputof converter |00. This simplexhigh quality signal could 'be Aradiate'd 'die rectlyto -the'receiving apparatus of? Figure 3"-'01' rel'ayed thereto through thev apparatus/oi Fig-l ure 2.

If'we assume thatthe-'high' quality channel A'is used-toproduce asi-n gl-e frequency mo du'la-tionythat sto fsay, directly-frequency modulate the radiL ated carrier as suggested'then thesignal to noise ratio' Y-as Icompared to ar'corresponding amplitude modulation system willr be" eq-ualfto thee'square roetV 'of 3-- multi-plied by the deviation ratio. I this case it will be A1so,`the sources 618" and 615" 

